In many situations, air within a building must be continually replaced for health and comfort reasons. Conditions such as these are frequently found in paint spray shops, foundries, chemical plants, welding shops, large restaurants, bowling alleys, warehouses, etc. However, taking in a large amount of ambient air, heating the air, and introducing it to the building can over burden existing heating systems. In such situations, a “makeup” air heater is often used to temper the incoming air, raising its temperature to the room temperature, and thus relieving the building heating plant from the extra load.
Makeup air units typically utilize either direct or indirect fire burners. In a direct fire system, the flame and its by-products are mixed directly with the incoming air stream and are added directly to the heated space. A heating process such as this does not require a heat exchanger and thus is more energy efficient than indirect fire systems. However, as the burner is located and operated directly in the air flow, typically within the existing duct work of the facility, the products of combustion are added directly to the heated space along with the heated air. Control of emissions is therefore of most concern. The oxygen needed for combustion in such systems is typically provided or generated by a fan or blower located downstream of the burner.
A direct fire burner is designed essentially from two main components: a gas manifold and air baffles. The gas manifold distributes gas evenly along the entire length of the burner. Air baffles are designed to create a combustion chamber and distribute a controlled amount of air into such a chamber. The baffles further serve to protect the flame from an excess supply of air, thus preventing the flame from being quenched.
In such units, the burner is typically positioned within an air duct proximate a profile opening. More specifically, a partition extends laterally across the air duct with the profile opening being provided centrally within the partition. The gas manifold and baffles of the burner are positioned so as to exhaust the flame and its combustion gases through the profile opening. The profile opening is designed to create a known pressure drop or velocity of air across the burner assembly. This velocity defines the operating range of the burner. If the pressure drop is too high or too low, the burner will not function properly. The proper size of the profile opening in such units is dictated by the total airflow through the unit and the size of the burner.
However, some systems are designed to deliver a variable air flow. In such units, where the total airflow delivered to the heated space changes, dampers are typically mounted adjacent the profile opening to adjust the effective size of the profile opening and thus the pressure drop across the burner. For example, at maximum airflow the dampers open and increase the overall size of the profile opening. Similarly, at minimum airflow the dampers close to decrease the overall size of the profile opening. Depending on the desired airflow, the dampers can be positioned anywhere in between fully open and fully closed.
While effective, such an approach is designed only to control airflow around the burner and to keep the burner operating per manufacturing instructions. No attempt is made to control the airflow downstream of the burner, nor is any attempt made to control emission output levels. Rather, the objective of such units is to provide a specific pressure drop across the burner to provide the combustion chamber with sufficient amounts of air at low to intermediate firing intensities to sustain proper combustion. At high firing intensities, such units rely on excessive air directed around the burner downstream of the profile opening, but by providing such an excess amount of unconditioned air downstream of the burner, emission output levels increase.
The most notable emission is nitrogen dioxide (NO2). Its production is the single most limiting factor in obtaining a high temperature rise in a direct-fire makeup unit in that firing intensity cannot simply be increased to a desired temperature rise if doing so results in undesirably high emission outputs. The current standards for acceptable nitrogen dioxide emission levels are regulated by statue. ANSI standards Z83.4 (non re-circulating direct gas fired industrial air heaters), and Z83.18 (re-circulating direct gas fired industrial air heaters) limit nitrogen dioxide emissions levels to 0.5 ppm (parts per million). The level of nitrogen dioxide emissions increases with temperature rise. The maximum temperature rise a direct fire heater can obtain is that temperature reached when nitrogen dioxide emissions levels, as they are currently regulated, are reached.
With a 0.5 ppm nitrogen dioxide emissions limit, a standard makeup air heater can typically achieve a maximum temperature rise of 100–120° F. (i.e., elevating the temperature of incoming air by 100 to 120° F.). To achieve higher temperature rise, for example, up to 140° F., manufacturers of makeup air units have reduced the overall air (measured in cubic feet per minute (cfm)) and gas (measured in British Thermal Units (BTU)) inputs. Since the emission of nitrogen dioxide is related to flame quenching and mixing of flames and their by-products with excess, cold, surrounding air, if the flame interaction with the air is limited, lower emission levels of nitrogen dioxide can be achieved. However, while such current systems can reach higher temperature rise due to slower air flow through the burner, and more uniform flow into the blower, the resulting burner is larger and more expensive than is desired, and takes longer to heat a given space due to the lower overall airflow.
It would therefore be desirable to provide such a direct air gas burner of a relatively compact inexpensive design, but which can provide greater air temperature rise for a given size, while still meeting current NO2 emissions regulations.